Numerous designs of passive and active automotive occupant restraint systems are known for enhancing occupant protection in the event of a vehicle impact. Passive systems such as inflatable restraints or air bags for frontal and side impacts and automatically deployed seat belt systems are known. Active seat belt systems have been used for many decades and are manually deployed by the occupant. The conventional seat belt system uses three points of connection with the vehicle structure and incorporates a lap belt section for engaging the occupant's lower torso, and a shoulder belt section for engaging the occupant's upper torso. When used, the seat belt restrains movement of the occupant in the event of a vehicle impact or rollover event. In order to enhance the comfort and convenience provided by the seat belt system, retractors are used which permit the belt webbing to be extracted from and retracted into the retractor, allowing movement of the occupant while maintaining the belt in close contact with the occupant. An inertia sensitive vehicle sensor which may be used with a webbing sensor, locks the retractor when an impact or rollover event is detected, preventing further extraction of webbing to restrain the occupant.
A more recent development in the area of seat belt systems is that of the so-called inflatable seat belt. An inflatable seat belt system incorporates a section of the shoulder belt which is inflated by a gas, typically by a pyrotechnic inflator, to increase its size and volume. Thus, when an inflatable seat belt is deployed, the normally narrow seat belt webbing expands significantly to reduce the contact pressure between the shoulder belt and the occupant during an impact event. Inflatable seat belts also act as a belt pretensioner by reducing belt slack. Inflatable seat belts are typically deployed using the crash sensing systems also used to deploy other inflatable restraint systems upon the detection of a vehicle impact or rollover event having predetermined dynamic characteristics.
Various approaches have been proposed to properly position the inflatable seat belt section with respect to the occupant's upper torso. One approach utilizes a pair of seat belt retractors, one having a spool for winding and storing the lap belt portion of the webbing, and a second retractor for winding and stowing the shoulder belt portion of the webbing. In addition to applications involving inflatable seat belt sections, dual retractor systems are also incorporated in certain vehicles to provide enhanced comfort and convenience for the vehicle occupants, and provides them with greater freedom of movement. This is particularly appropriate in luxury vehicles where the additional costs for the benefits of a dual retractor system are acceptable.
Providing dual retractors for the seat occupants increases the cost and complexity of the restraint system. Further compounding the cost duplication of a dual retractor system is the emergence of an increasing number of sophisticated control systems for retractors. An example of such a control system is a retractor pretensioner. Retractor pretensioners cinch the seat belt webbing against the occupant, eliminating slack immediately upon the detection of a vehicle impact. Reducing the slack in the seat belt system enables the occupant's forward motion to quickly engage the seat belt to begin dissipating impact energy. Various designs of pretensioners are presently known, including a type known as a Roto-Pretensioner which incorporates a series of balls in a gas duct which are driven by the deployment of a micro gas generator to engage with and wind the retractor spool sprocket to retract the webbing.
Another retractor control device presently considered are so-called pre-pretensioners. These devices begin to cinch the seat belt webbing against the occupant even before an actual impact is detected. This can be provided using, for example, radar, ultrasonic, or other sensing systems which anticipate a vehicle crash event before actual impact has occurred. Electronically controlled retractors are also being developed in which the locking function is controlled by a solenoid, as opposed to the more prevalent inertia sensitive rolling ball or pendulum type locking devices. Other electronic controlled designs of these devices are also available involving the use electrodynamic devices to cause locking or even actively winding the seat belt retractors as desired. The implementation of these retractor control systems further increases the cost of providing such features in a dual retractor system, since these functions may be duplicated for each retractor.